The precise positioning of the read and write heads over tracks on rotating disks is achieved by a servo system in disk drive using servo patterns that are prerecorded in magnetic material on the disks as part of the manufacturing process. Typically servo patterns are written at regular angular intervals and form wedge-shaped areas extending from the inner diameter (ID) to the outer diameter (OD) that define the sectors in the concentric tracks. The servo fields for each sector typically include a servo address mark (SAM), which is a unique pattern in the servo area that is used to synchronize processing of the subsequent servo fields.
SAM detection window is the allowable time window within which the read channel attempts to locate the SAM. To avoid misdetection, it is better to have a shorter time window and have the beginning and end of the window be very precise. In U.S. Pat. No. 6,021,012 to H. Bang (Feb. 1, 2000) an automatic servo address mark detection and servo timing compensating circuit is described that includes a count control signal where the servo address mark is not detected within a range of a servo address mark window.
A variety of clock signals are used in a disk drive and are affected by power management modes. U.S. Pat. No. 7,739,533 to Rauschmayer, et al. (Jun. 15, 2010) describes an operational power management system for a disk drive that includes two or more clock sources and a power management controller.
In U.S. Pat. No. 8,189,285 to Spaur, et al. (May 29, 2012) automatic time base adjustment for disk drive servo controllers is described. A servo timer is adjusted between the consecutive servo fields based on the skew value that occurs when the system switches from reading with one head to reading with another head (a head change).
Typically the system clock is based on oscillating crystal and PLL electronics and requires relatively low power. The system clock runs at both startup of disk drive and also during power-saving sleep mode of hard drive. The system clock operates independently from the rate that signals to and from the rotating disk are actually written and read. Because the system clock does not vary with disk speed, it is less useful for detection of SAMs. The disk lock clock (DLC) or disk synchronous write clock (DSW) is a clock signal that is synchronized to signals generated as servo address marks (SAMs) pass under the read head during operation. The DSW is more useful to identify mis-detections (early or late) and missing SAMs but requires more electronics being active and accordingly more power usage. The SAM signal from the read head must be processed by the read channel portion of the system electronics and, therefore, the DSW is not available when read channel and/or read-head current is shut down in power-saving modes. Various systems and methods for power management are disclosed in the prior art including a sleep mode in which the read channel is turned off independently from other parts of the system. Thus, servo system operation in power-saving modes becomes a challenge.
In the prior art it is difficult to control servo gate, SAM window, channel power save mode, and arm electronics preamp power save mode precisely without long term jitter and errors.